This invention relates to a video signal processing circuit for producing a dissolve signal to control dissolving of a video signal.
In the prior art, a dissolve between first and second video sources is performed by applying a ramp control signal to a differential amplifier. The ramp signal proportionately increases the gain of the differential amplifier for one video signal and decreases it for the other. By adjusting the slope of the ramp control signal, the desired duration of dissolve is achieved.
The ramp control signal may be generated in an analogue circuit using resistors and capacitors to generate the basic ramp and an amplifier having controllable gain to adjust the slope of the ramp and hence the duration of the dissolve. In addition, a ramp generator has been used based on an n-bit digital counter which counts a fixed frequency such as the frame frequency. The digital number stored in the n-bit digital counter is D-A converted. The resulting analogue output signal is amplified by an amplifier whose gain is adjusted to give the desired duration. In order to achieve a long duration, n must be large. This increases the cost of the device.
Furthermore, to obtain a wide range of durations, the gain of the amplifier must be controllable over a very wide range. This gives difficulty in generating a steep ramp due to problems in maintaining linearity. The shallowness of the minimum slope is limited by the size of n.